1. Field of the Invention
The present invention relates generally to multilayer paper products. More specifically, it relates to improved processes for producing multilayer papers having high surface smoothness coupled with improved stiffness.
2. Description of the Prior Art
The principal raw material used in paper manufacture is fiber derived from wood. The fibers are separated from the wood by a chemical or mechanical defiberizing process. The fibrous material obtained by the chemical method is generally called chemical pulp, while the fibrous material produced mechanically is called mechanical pulp.
In a paper manufacturing process, the fibers are suspended in water to form a dilute fiber/water suspension that is then passed over a paper machine to form paper.
For most paper mills, the furnish of raw materials is economically limited to use of available woods within the immediately surrounding area. Many mills utilize both softwoods and hardwoods, the percentage of each used varying depending upon the mill's location. An additional reason for the use of fiber mixtures is that different fibers give the paper different properties. Thus, some fibers give the paper increased strength, while other fiber types may improve other properties, e.g., brightness, smoothness, opacity, or porosity. As a result, there are numerous fiber combinations used to manufacture the various kinds of paper.
Recently, the paper industry has encountered several serious problems. The cost of wood pulp has increased. In addition, the energy cost of paper manufacturing has been increasing. These circumstances have placed the paper industry and its customers in a situation of having to make a choice. Either the higher costs must be paid for, or fibers of lesser quality must be utilized. To avoid the higher costs while using present paper manufacturing techniques, some deterioration of the quality of the paper products resulted, in particular the printing properties. One response to these problems in the industry as a whole has been the development of multilayer production techniques. Multilayer techniques were first introduced in the production of paperboard. It was soon realized that this technique permitted the placing of different types of pulp in the different layers in order to optimize the usage of the different furnishes. Structured web forming is now an established concept for board and tissue products. For example, linerboard is manufactured in a two-layer structure. The motivation for this was economic--both low cost fibers and waste could be placed in the bottom sheet, while virgin fibers could be placed in the top sheet where appearance is important. Multilayer techniques, however, have not been developed for use in manufacturing fine printing grade papers.
As mentioned, such previous use of multilayer technology has been motivated by several considerations. The foremost consideration has been economics. Multilayer technology has been used to allow lower cost materials, such as chemithermomechanical pulps (CTMP) and waste, to be hidden in the inner layer. An additional advantage has been that property improvements have been realized by putting materials where they will be most advantageous to end use, rather than mixing them randomly. Another example of this is the improvement in stiffness that comes from putting a bulky middle layer between two layers of virgin chemical pulp. Use of multilayer techniques has also allowed the papermaker some extra degrees of freedom to separately treat the layers and achieve superior properties compared to what would be achieved if all of the furnish were uniformly processed.
Another example of multilayer technology is the segregation of hardwood and softwood in tissue to put the softer, hardwood pulp on the outside of the sheet where the consumer will touch it, and the stronger, softwood pulp in the inner layer.
The physical properties of multilayer paper can be divided into two categories. Some properties, such as tensile, tear, burst, density, and opacity, obey the law of mixtures and will be the same for sheets made either with a homogeneously mixed furnish or a three-layer structure with furnish components segregated. For these properties, there should be no intrinsic advantage to making a three-layer sheet. Other properties, however, such as bending stiffness, folding endurance, brightness, smoothness, surface compressibility, and printability, can be different in a three-layer sheet from what is observed in a sheet made from the same furnish homogeneously mixed and will affect the production of printing grade papers.
Bending stiffness increases can be obtained with a multilayer sheet when the weaker, lower density component is concentrated in the inner layer and the higher strength, higher density component is concentrated in the outer layers.
The prior art also teaches that the surface properties and printability of multilayer papers are determined by the outer-layer fibers. It is known that the smoothness and printability are directly related to a fiber property known as coarseness. Coarseness is a measure of weight per unit length, and it reflects the fiber diameter and cell wall thickness and density. The reciprocal of coarseness is sometimes referred to as fineness. Therefore, the coarseness or roughness of the fibers in the outer layer of a multilayer sheet has been generally predicted to determine the smoothness and printability of that sheet. See e.g., J. A. Bristow and N. Pauler, "Multilayer Structures in Printing Papers," 1983 SVENSK PAPPERSTIDNING R 164 at R 168-69. In Bristow and Pauler, multilayer sheets were manufactured using chemical pulp in certain layers and mechanical pulp in others. No particular tests were performed to examine the effects of using different types of raw materials as the starting material for a multilayer sheet made entirely from chemical pulp.
Compressibility can also affect printability properties. It has been seen that mechanical pulps are typically more compressible and that a multilayer structure, with the mechanical pulp in the outer layers and chemical pulp in the center layer, shows compressibility and printability more similar to an all-mechanical pulp sheet than to an all chemical pulp sheet.
As discussed earlier, the fiber furnish used in paper making is often composed of more than one fiber component. Thus, it is known that in multilayer technology improved stiffness can be realized, compared to a homogenous mixture, by putting the stronger, denser, higher modulus fibers in the outer layer, and the weaker, lower density pulp in the inner layer. In certain instances, the stronger fibers are also coarser than the weaker fibers in a particular furnish. When this occurs, according to the prior art observations and predictions, there is a property tradeoff: putting fibers that are stronger and coarser in the outer layer and fibers that are weaker and finer in the inner layer yields a multilayer sheet with improved stiffness, but with poorer smoothness and printability. Conversely, placing the finer (less coarse) fibers in the outer layer gives improved smoothness, but poorer stiffness. Thus, it appears that multilayer sheets made with high basis weights of coarse fibers in the outer layer have poor smoothness and printability. As a result of this strength/smoothness trade-off, there has been no incentive to manufacture printing papers in this manner.
This is true, particularly dealing with papers for letterpress and gravure printing, where surface smoothness is a critical concern. A more limited degree of smoothness is also required for the offset and flexographic processes in which a flexible printing form is used. Smoothness is required because the depressions in rough sheets are not covered with ink, resulting in either speckle in solid printed areas or a lack of definition in halftones. Many other attributes of print quality are important, but if a print has poor coverage, its other features will be largely ignored.
At the same time, the producers of printing papers have been challenged to produce smooth sheets at higher bulk. The trend to lighter basis weight papers has emphasized the need for high bulk in order to maintain stiffness. Nevertheless, these papers must still retain good smoothness characteristics in order to print well.
Technical advances in paper machine design have now made it possible to use multilayer structures not only in paperboard but also in thinner paper such as newsprint, fine papers and tissues. See e.g. J. A. Bristow and N. Pauler, "Multilayer Structures in Printing Papers," 1983 SVENSK PAPPERSTIDNING R 164, discussing the use of chemical and mechanical pulps in alternate layers.
In U.S. Pat. No. 4,781,793, issued to Halme, entitled "Method for Improving Paper Properties Using Long and Short Fiber Layers," there is disclosed a method for forming a sheet of paper with a predominance of long fibers in an outer surface and finer fibers in the center. The method which is disclosed is comprised of forming a base furnish and then separating the furnish into components, one of which contains a predominance of long fibers, but which also contains short fibers, and the other which contains a predominance of short fibers, but which still would contain long fibers, fillers and fines, etc. The use of the fiber mixtures, that is the long and short fiber components, is stated to help the retention and also to improve certain paper properties. The furnishes which are used are disclosed to be made of a chemical pulp for the short fibers and a mechanical pulp for the long fibers. While the layers may be different, each is to some extent a composite of both types of fibers, that is long and short fibers.
In U.S. Pat. No. 2,881,669, issued to Thompson et al., entitled "Paper or Board Products," there is described a paper or board product which is stated to have long fibers predominantly on opposite sides of a short fiber inner zone. This is stated to be accomplished as a result of the inherent drainage characteristics of the papermaking machine, wherein the long fibers tend to be retained when the papermaking machine forms the initial surface, and then the shorter fibers, and in addition long fibers, are also collected on the initial long-fiber layer. The resultant paper therefore has a graduated structure of predominantly long fibers at the outer surface and predominantly shorter fibers in the inner portion. The paper does not, however, have a definite multilayer structure with coarse fibers on the outer surface and fine fibers in the interior.
Another patent, U.S. Pat. No. 4,888,092, issued to Prusas et al., discloses a three-ply sheet, wherein the outer plies are made up of fines in order to improve surface smoothness.
Nevertheless, the problem of overcoming the trade-offs between strength and smoothness between various starting pulps remains. Accordingly, there exists a need for a method to produce products having improved stiffness characteristics while maintaining high quality smoothness and printability characteristics.